1. Field of the Invention
The present invention relates to a charged particle beam writing apparatus and a charged particle beam writing method. For example, it relates to a writing apparatus and a writing method used when performing multiple writing with an electron beam.
2. Description of Related Art
The microlithography technique which advances microminiaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in the semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. In order to form a desired circuit pattern on semiconductor devices, a master or “original” pattern (also called a mask or a reticle) of high precision is needed. Thus, the electron beam writing technique, which intrinsically has excellent resolution, is used for producing such a highly precise master pattern.
FIG. 14 is a schematic diagram for explaining operations of a variable-shaped electron beam (EB) writing apparatus. As shown in the figure, the variable-shaped electron beam writing apparatus operates as described below. A first aperture plate 410 has a quadrangular, such as a rectangular, opening 411 for shaping an electron beam 330. A second aperture plate 420 has a variable-shape opening 421 for shaping the electron beam 330 having passed through the opening 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to pass through a part of the variable-shape opening 421 of the second aperture plate 410, and thereby to irradiate a target workpiece or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g. X direction) during the writing. In other words, a quadrangular shape that can pass through both the opening 411 and the variable-shape opening 421 is used for pattern writing in a writing area of the target workpiece 340 on the stage continuously moving in the X direction. This method of forming a given shape by letting beams pass through both the opening 411 of the first aperture plate 410 and the variable-shape opening 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) method.
In the electron beam writing, it is expected to shorten a writing time. For shortening a writing time, there is a method of writing with a beam of high current density. However, if a beam of high current density irradiates a resist-applied mask, a problem referred to as “resist heating” may occur that the temperature of the resist raises and dimensions different from desired ones are formed. Furthermore, if the temperature of the resist raises to exceed a resist melting point, a problem may also occur that the resist evaporates and disperses to pollute the inside of the writing chamber. Therefore, in the electron beam writing, a dose per shot of a figure to be written is reduced by employing a multiple writing system that overlappingly writes a writing pattern a plurality of times. Thereby, the temperature raise of resist is diminished. With regard to the multiple writing system, there are some methods proposed: one is that the first writing and the second writing are performed per stripe area made by virtually dividing a chip area or a writing group area composed of a plurality of chips under the same writing conditions, and another is that the writing operation is advanced while alternately performing the first writing and the second writing per subfield in a stripe area (refer to e.g., Japanese Patent Application Laid-open (JP-A) No. 2008-117871).
When performing writing of figures to be written, since a proximity effect etc is corrected by a dose modulation method, the suitable dose varies depending upon a figure to be written. Conventionally, treating a maximum dose in doses necessary for writing figures in a chip area or a writing group area as a reference dose, writing of the entire chip area or writing group area is performed with a multiplicity capable of obtaining the maximum dose. Therefore, depending on a figure written, there is a case of performing multiple writing too many times than needed. Thus, there has been a problem that the writing time is prolonged by the number of times of such an excessive multiple writing.
In the current tendency of expecting to shorten the writing time, if there exists a figure for which too many times of multiple writing than needed is performed, as described above, thereby causing a problem that writing time is prolonged by the number of excessive shots for the figure.